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        <p>当粒子量级达到一定数量的时候，性能开始降低，帧率开始下降，这是我们不想看到的，因为这很影响用户体验。</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;!DOCTYPE <span class="meta-keyword">html</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">html</span> <span class="attr">lang</span>=<span class="string">&quot;en&quot;</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">head</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">meta</span> <span class="attr">charset</span>=<span class="string">&quot;UTF-8&quot;</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">meta</span> <span class="attr">name</span>=<span class="string">&quot;viewport&quot;</span> <span class="attr">content</span>=<span class="string">&quot;width=device-width, initial-scale=1.0&quot;</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">title</span>&gt;</span>Document<span class="tag">&lt;/<span class="name">title</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">link</span> <span class="attr">href</span>=<span class="string">&quot;./style.css&quot;</span> <span class="attr">rel</span>=<span class="string">&quot;stylesheet&quot;</span> <span class="attr">type</span>=<span class="string">&quot;text/css&quot;</span> /&gt;</span> </span><br><span class="line"><span class="tag">&lt;/<span class="name">head</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">body</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">canvas</span> <span class="attr">class</span>=<span class="string">&#x27;canvas&#x27;</span> <span class="attr">id</span>=<span class="string">&#x27;canvas&#x27;</span> &gt;</span></span><br><span class="line">        您的浏览器不支持canvas</span><br><span class="line">        <span class="comment">&lt;!-- 如果浏览器不支持canvas则显示标签中的文字 --&gt;</span></span><br><span class="line">    <span class="tag">&lt;/<span class="name">canvas</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">canvas</span> <span class="attr">class</span>=<span class="string">&#x27;canvas&#x27;</span> <span class="attr">id</span>=<span class="string">&#x27;offCanvas&#x27;</span> &gt;</span></span><br><span class="line">        您的浏览器不支持canvas</span><br><span class="line">        <span class="comment">&lt;!-- 如果浏览器不支持canvas则显示标签中的文字 --&gt;</span></span><br><span class="line">    <span class="tag">&lt;/<span class="name">canvas</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">script</span> <span class="attr">type</span>=<span class="string">&#x27;text/javascript&#x27;</span> <span class="attr">src</span>=<span class="string">&#x27;./script.js&#x27;</span>&gt;</span><span class="tag">&lt;/<span class="name">script</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">body</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">html</span>&gt;</span></span><br></pre></td></tr></table></figure>

<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/** <span class="doctag">@type <span class="type">&#123;HTMLCanvasElement&#125;</span> </span>*/</span></span><br><span class="line"><span class="keyword">const</span> canvas = <span class="built_in">document</span>.getElementById(<span class="string">&#x27;canvas&#x27;</span>)</span><br><span class="line"><span class="keyword">const</span> ctx = canvas.getContext(<span class="string">&#x27;2d&#x27;</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> offCanvas = <span class="built_in">document</span>.getElementById(<span class="string">&#x27;offCanvas&#x27;</span>)</span><br><span class="line"><span class="keyword">const</span> offCtx = offCanvas.getContext(<span class="string">&#x27;2d&#x27;</span>)</span><br><span class="line"></span><br><span class="line">canvas.width = <span class="number">600</span>;</span><br><span class="line">canvas.height = <span class="number">600</span>;</span><br><span class="line">offCanvas.width = <span class="number">600</span>;</span><br><span class="line">offCanvas.height = <span class="number">600</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> posx = <span class="number">0</span>,</span><br><span class="line">    posy = <span class="number">0</span>,</span><br><span class="line">    isMouseInRect = <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 将复杂的绘画操作，画在离屏canvas上</span></span><br><span class="line"><span class="keyword">const</span> drawAlot = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i &lt; canvas.width; i += <span class="number">10</span>) &#123;</span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">let</span> j = <span class="number">0</span>; j &lt; canvas.height; j += <span class="number">10</span>) &#123;</span><br><span class="line">            offCtx.beginPath();</span><br><span class="line">            offCtx.arc(i, j, <span class="number">5</span>, <span class="number">0</span>, <span class="number">2</span> * <span class="built_in">Math</span>.PI, <span class="literal">true</span>)</span><br><span class="line">            offCtx.stroke()</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">canvas.onmousemove = <span class="function">(<span class="params">e</span>) =&gt;</span> &#123;</span><br><span class="line">    <span class="keyword">const</span> mouseX = e.offsetX,</span><br><span class="line">        mouseY = e.offsetY;</span><br><span class="line">    <span class="keyword">if</span> (mouseX &gt; posx &amp;&amp; mouseX &lt; posx + <span class="number">50</span> &amp;&amp; mouseY &gt; posy &amp;&amp; mouseY &lt; posy + <span class="number">50</span>) &#123;</span><br><span class="line">        isMouseInRect = <span class="literal">true</span></span><br><span class="line">    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">        isMouseInRect = <span class="literal">false</span></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">setInterval</span>(<span class="function">() =&gt;</span> &#123;</span><br><span class="line">    <span class="keyword">if</span> (!isMouseInRect) &#123;</span><br><span class="line">        posx += <span class="number">10</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    ctx.clearRect(<span class="number">0</span>, <span class="number">0</span>, canvas.width, canvas.height)</span><br><span class="line">    <span class="comment">// 真正要用到复杂的绘画的时候，直接从离屏canvas上拷贝过来</span></span><br><span class="line">    ctx.drawImage(offCanvas, <span class="number">0</span>, <span class="number">0</span>, offCanvas.width, offCanvas.height, <span class="number">0</span>, <span class="number">0</span>, canvas.width, canvas.height)</span><br><span class="line">    ctx.fillRect(posx, posy, <span class="number">50</span>, <span class="number">50</span>);</span><br><span class="line">    <span class="keyword">if</span> (posx &gt;= canvas.width) &#123;</span><br><span class="line">        posx = <span class="number">-50</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;, <span class="number">100</span>);</span><br><span class="line"></span><br><span class="line">drawAlot()</span><br></pre></td></tr></table></figure>


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